1. Field of the Invention
The present invention relates generally to methods and compositions for preventing precipitation of iron compounds during acid treatments of wells. In particular, this invention relates to methods and compositions for catalyzing the rate of iron reduction during acid treatment of wells. Specifically, this invention relates to a method for accelerating the reduction of ferric ions utilizing antimony ions in combination with other materials to minimize precipitation and other complications which result from iron compounds during acid treatments. Such other materials may include a source of copper ions and at least one of phosphinic acid, salt of phosphinic acid, or a mixture thereof.
2. Description of Related Art
During well treatments and related operations employing acid, contamination of acid by dissolved iron or iron compounds is a known phenomenon. Contamination of a well treating acid by dissolved iron or by iron compounds during the process of acid treating a well bore and/or subterranean formation is almost inevitable. In the treatment of sour wells ferric iron may oxidize sulfides to insoluble elemental sulfur deposits, and ferrous iron can form ferrous sulfides as the acid spends. These materials may cause well plugging. A discussion of dissolved iron problems and previous methods for addressing these problems may be found in Canadian Patent No. 1278178 and U.S. Pat. No. 5,063,997.
As described in the above-mentioned references, it has been estimated that in the absence of an acid prewash levels of 9,000 to 100,000 mg/L of dissolved iron may occur. It has been reported that a source of iron is the mill scale and rust on the steel tubulars used during stimulation in production applications. If an acid wash treatment is carried out prior to a formation treatment, it has been reported that levels of dissolved iron entering a formation will typically be in the range of 1,000 to 2,000 mg/L. Other reports have indicated that small volumes of acid wash may result in iron levels of 500 to 7,000 mg/L contacting the formation. Additional complications such as disposal problems, low reservoir pressure or the presence of a permanent packer around the tubing may make it impossible to conduct an acid wash prior to the acid treatment. Thus, it may be very hard or impossible to reduce levels of dissolved iron to acceptable levels.
As described in the above-mentioned references, many attempts have been made related to reduction of ferric hydroxide precipitation. Such attempts have included sequestering of ferric acid in acid solution using salicylic acid or sulfosalicylic acid proposed. However, sulfosalicylic acid has been found ineffective in preventing iron asphaltene sludge in 15% hydrochloric acid. Use of sequestering agents such as citric acid, ethylene diamine tetra-acetic acid (EDTA) or nitrilo triacetic acid (NTA) has been described. However, effectiveness of such materials at temperatures above 125-250.degree. F. is poor. Other compounds which have been described include ascorbic acid, erythorbic acid and/or their salts. However, tests have shown that effectiveness of erythorbates at preventing iron precipitates drops off rapidly as hydrochloric acid strength increases to 15%. Furthermore erythorbates are unstable in hydrochloric acid and degrade fairly rapidly to solids. The rate of degradation increases dramatically at higher temperatures, raising particular problems for treatments in which acid remains at reservoir temperatures for several hours, such as staged acid treatments.
Many reducing agents such as stannous ion, hydrazine and related compounds etc. may cause asphaltic sludge even in the absence of irons. Other reducing agents such as erythorbates, and most organic compounds, are typically degraded to varying degrees in strong acid. This degradation may form carbonaceous residues which are ineffective in controlling gradual contamination by iron. Materials such as thiosulphates may degrade in acid to form elemental sulphur precipitates, and are thereof undesirable. An alternative to these reducing agents consists of phosphinic acid (hypophosphorous acid) and/or salts of phosphinic acid.